Halide Design DAC HD D/A converter

When it comes to getting audio from a PC via its USB port, the buzzword du jour is asynchronous. This cryptic term refers to which device has control over the timing of the audio data being streamed from the computer: the computer itself, or the device receiving the data. It might seem logical to have the computer control the timing, but this is not so. When digital audio data are converted to analog by a D/A converter, control over exactly when each dataword is converted is critical for the best quality of sound. Any uncertainty in that timing manifests itself as analog distortion, aka jitter.

It is therefore good engineering practice to place the timing circuit, for obvious reasons referred to as a "clock," as close as possible to the D/A circuit. But a PC clock several feet away, at the other end of a USB cable, is clearly not "as close as possible." Worse, a computer must attend to other tasks, so its clock is inherently less stable than the crystal oscillator found in a DAC. The optimal solution is to allow the DAC to control the flow of USB data from the computer; this is what is meant by asynchronous.

Not all USB receiver chips can be operated in asynchronous mode, an exception being the Texas Instruments TAS1020B. This USB 1.1 device includes an onboard microcontroller that can be programmed to run the interface in asynchronous mode. Gordon Rankin of Wavelength wrote proprietary code for the TAS1020B's microcontroller; as well as using it in his own USB products, he licenses this Streamlength code to hardware manufacturers. One such is Halide Design, who used Streamlength in their S/PDIF Bridge converter ($395), which I reviewed in December 2010. The Bridge comprises a USB cable with a tubular housing at one end, terminated in either an RCA or a BNC jack. The Bridge operates in asynchronous mode to convert USB-streamed audio data to the S/PDIF serial format required by traditional D/A processors. Because there is no jitter-inducing S/PDIF cablethe Bridge connects directly to the processor's input jackthe Bridge became my preferred means of connecting a computer to DACs that lacked an asynchronous USB port.

It seemed an obvious development for Halide to marry the Bridge's asynchronous USB receiver to a DAC chip, to create a complete USB/analog converter (footnote 1). This they have done with the subject of this review, the DAC HD. Initially priced at $550, the DAC HD will be available for $495 by the time you read this review.

Asynchronous Conversion
Like the Bridge, the DAC HD includes captive cables, in this case a black version of Wireworld's Starlight USB cable, which features silver-clad conductors and a unique geometry said to allow use in lengths up to 7m. (A 2m cable is standard; 7m and other lengths are available as options when ordering.) Analog output is via two 6" lengths of silver-conductored cable terminated in Eichmann silver Bullet RCA plugs. (These RCAs are favorites of mine because they make the ground before the signal connectiona commendable feature.)

The circuitry, all surface-mount, is contained in a small (1.875 cubic inches), black-anodized, machined-aluminum enclosure, and is carried on a small double-sided printed circuit board. Power is taken from the 5V USB bus and first filtered (two-pole passive), then regulated; there are eight regulators and separate power rails for the digital circuitry, the master clock, and the output stage. The TAS1020B is mounted behind the USB port and feeds a Wolfson WM8716 DAC chipa two-channel, multi-bit, sigma-delta, 24-bit type. Although the WM8716 can operate at sample rates up to 192kHz, the TAS1020B handles sample rates only up to 96kHz, including the important 88.2kHz rate. The Wolfson DAC includes fast- and slow-rolloff digital reconstruction filters, but also can be used with an external filter. The output stage uses high-performance op-amps.

No driver programs are required with either Macs or PCs. The user simply plugs the DAC HD into one of the computer's USB ports and selects it as the preferred audio output device.

Sonics
As I was about to start this review, I retired my 2006-vintage G4 Mac mini and replaced it with a 2.7GHz, i7-powered Mac mini with 8GB of RAM. Both the Mac mini and the Pure Music program recognized the Halide when I plugged it in, and I set it as the default playback device. Pure Music was set to upsample CD-sourced files to 88.2kHz with its Maximum Fidelity sample-rate converter.

Halide Design states that the DAC HD is cryogenically treated to give a "smoother, more resolved sound." Whether or not that was the reason, my first impression was indeed of a very smooth sound. There wasn't quite the wealth of recorded fine detail that I'm used to hearing with the megabucks D/A converters that usually grace my system, nor was there quite as expansive a soundstage. In my recording of Carol Wincenc performing Mozart's Flute Quartet in D, K.285, at the Santa Fe Chamber Music Festival (ALAC file sourced from CD: Serenade, Stereophile STPH009-2), while the flute was unambiguously placed in front of the string trio, the acoustic of St. Francis Auditorium sounded a little smaller and drier than it should. But that was a minor criticism, considering that when I rose from the listening chair to find my notebook, I realized I had listened to all three movements of the quartet. There was something very seductive about the sound of the DAC HD.

Footnote 1: Halide Design's earlier, less expensive Devilsound DAC uses the Burr-Brown PCM2706 USB receiver, which operates in the more conventional adaptive isochronous mode, in which the computer controls the timing of the data flow.

your review does not address the Halide HD’s inability to control the volume from the computer without derogating the sound.

if you are feeding the Halide's output to your preamplifier, you don't need to adjust the DAC HD's volume with the computer, so your criticism is moot. If you are feeding the Halide drectly to a power amplifier or headphone amplifier without a volume control, then you can use the volume control in Pure Music, which is very much more transparent than the system volume control. But I don't recommend this set-up, as any reset of the system might result in full-level sound being fed to your speakers.

With all the things you here about quality of good components weighing more, it surprising the review of the Halide compared to Musical Fidelity. I would treat it more as an accessory if I were in and out of town. Hey a lot of guys are trying to fill up our shelves on our entertainment center. [Hiding real prices from our wives?] The cover to the book matters. Good Citizen Swallow is a neat low energy track to test a component though. I really got into into it.

I don't see why. Both products are intended to do exactly the same thing: take the bits from an audio file, convert them to analog, and send the signal to a line preamplifier. The Halide DAC HD turns a PC into a genuine high-end source at an affordable price.

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Good Citizen Swallow is a neat low energy track to test a component though. I really got into into it.

The "Lofty Fake Anagram" album was one of the first LPs I bought 40 years ago that sounded better as I improved my system.

Having recently acquired a FiiO E17, an HRT iStreamer, HRT Headstreamer, Audioengine D1 and Audioquest Dragonfly, I learned about some of the features and sound quality differences of these items. Since the Dragonfly was nearly twice the price of the Headstreamer, I expected it to sound much better, but it doesn't. It may be better, but not anywhere near the difference between it and the lower fidelity FiiO E17 and iStreamer.

Since the latter 3 DACs I noted all sound similar in quality, I wouldn't expect the DAC-HD to sound a lot better. There does seem to be a glut of these mini-DACs coming on the market now, which may be due to widespread availability of the key components. An interesting and useful feature of many or most of these mini-DACs is the ability to drive average hi-fi headphones directly at satisfactory volume, and while the manufacturers are careful to not refer to that feature as a 'headphone amp', it begs that comparison.

And that utility to drive hi-fi headphones raises the volume control issue as one other person noted here. I've read the FAQs, many reviews, and discussed the volume issues with some audiophiles, but it's still not very clear who or what is doing the volume attenuation at all times. Despite some of the DAC makers' claims about 'repurposing' a system volume control, it just seems to me that volume must be part of the digital data that goes to the DAC from the computer, and so the DAC's output volume would somehow be set according to the data bits from the computer that describe a volume setting. At the very least, a more precise description of that process by the manufacturers would clear things up. A 'for example' text (which is just an example, not intended to be an actual description) might read like this: "The computer music player senses its volume setting and the system volume setting and internally decides which is applicable (or combines both), and then sends that setting as part of the data stream going to the USB port, via the specified or default output device. It is up to the device on the USB port to act on that setting or ignore it altogether."

An interesting and useful feature of many or most of these mini-DACs is the ability to drive average hi-fi headphones directly at satisfactory volume, and while the manufacturers are careful to not refer to that feature as a 'headphone amp', it begs that comparison.

The engineering requirements for a true headphone amplifier and a DAC that is intended to drive a line-level input are different. So if a manufacturer refrains from claiming that their mini-DAC is also a headphone amp, I feel that should be respected.

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And that utility to drive hi-fi headphones raises the volume control issue as one other person noted here. I've read the FAQs, many reviews, and discussed the volume issues with some audiophiles, but it's still not very clear who or what is doing the volume attenuation at all times.

If the system volume control is set to the maximum, then the bits sent to the DAC are the bits in the file, which is the optimal situation - no loss of resolution. The Halide DAC HD doesn't have any volume control; the AudioQuest DragonFly (reviewed in our October issue) has an analog-domain volume control, allowing it to be used as a headphone amp without loss of resolution. The same is true for the CEntrance DACPort.

If you want to use the Halide as a headphone amp, then you can either use the system volume control, which will degrade sound quality, or use the high-precision software volume control provided by something like Pure Music, which does a lot better at preserving sound quality. But the real solution is _not_ to use the Halide as a headphone amp - controlling the volume with the computer eliminates the sound quality advantage it has over less-expensive products like the DragonFly.

it just seems to me that volume must be part of the digital data that goes to the DAC from the computer, and so the DAC's output volume would somehow be set according to the data bits from the computer that describe a volume setting.

If we consider that a digital level of 0db is the maximum, full distortionless output, that figure is represented by such things as iTunes' maximum volume setting and other software players maximum volume settings. This reference is taken from classic VU meters which display 0 as the max and a few positive integers "beyond" that as the permissible headroom analog recording can sometimes afford. Digital of course can't do that.

Since a digital recording has a relative volume level when created, that becomes the basis or reference for its noise level for whatever resolution was used to create it.

Say you have a 16-bit file played "normally"---fully. Your software player's volume control is maxed. Remember, doing so doesn't "amplify" anything; the volume control is out of the circuit if you will and all 16 bits arrive at the DAC.

Now lower the volume in iTunes, Pure Music, foobar2000---doesn't matter. You've just reduced the number of bits delivered to the DAC because that's the only way the software CAN reduce the relative volume of a given file.

Whenever you lose bits, you gotta dither (add sophisticated noise) to mask the effects of the lost bits, which by themselves would likely be worse.

iTunes now dithers-I think-but it won't be as sophisticated as the dithering Pure Music and some others can apply. Pure Music even says that if you want to use its most sophisticated dither you need a more recent Mac --- applying good dither on the fly, i.e. while the file is playing, doesn't come free and you need CPU cycles.

Analog volume controls have their own problems of course but traditionally are more benign. That's why you leave your software's volume maxed (think of it as delivering "line level" to your amp) and control volume as you always have, at your amp. The output levels from one DAC's analog out to the next likely isn't too great, just as it isn't likely too different from one CD player to the next.

The Dragonfly and Headstreamer, both being headphone-oriented but with an audiophile conscience, include digitally-controlled analog volume control. Something on the computer (music player app?) retains its volume control interface but instead of delivering a lower relative volume to the DAC delivers full volume and invokes an analog volume within the Dragonfly/Headstreamer hardware. Don't ask me how any of that works.

deckeda :> Quit the music player and the computer acts like it always has.

The Dac only acts on the music player. It has no effect on MOG, Pandora, Netflix, Hulu, Google phone or any other audio source. Even thou everything has to pass through Halide HD to get from the USB to the pre-amp?

When we say the DAC acts only on the music player, is that because the music player has selected a specific driver or sound processor to send its output to the DAC, whereas the rest of the system isn't calling that same driver/sound processor for its output for the other services?

Only dedicated music players will provide the ability to "link up" with an external DAC. A web browser or Spotify app for example has no understanding of what an external DAC is ...

The other way to do it (i.e., send everything through the external DAC) would be to setup the computer OS' audio out to use the DAC by default. And you still would need to setup the software player to use it (which is no big deal.)

Optical connectors and cable such as TOSLink has a physical/mechanical advantage of being immune to electrical interference and other issues like capacitance, voltage loss etc.

What it gains are disadvanatges in suspectibility to other physical attributes such as getting pinched (!) and whatever else challenges light transmission. And it still has a physical length limitation before it begins to lose connectivity with whatever's next in the chain.

The practical issues however, are that for a given DAC that accepts both optical and electrical inputs, one may be more susectible to jitter than the other, or react with a given cable more than the other, leading to performace differences between the two inputs. Numbers ain't numbers.

There are also often resolution differences. Some DACS or computer cards will limit their TOSLink to 24/96. Others may put that limitation on their digital coax port but not the TOSLink. And so on.

And all of that assumes the computer is delivering "perfect numbers" out, which it may not if it's RAM-starved, the storage HD is being asked to do two things at once, the CPU is overburdened trying to decode a lossless file while playing and doing something else, etc.

I have the Halide DAC HD in my computer audio system and I`m not clear on this discussion regarding volume control. I turn everything on and listen to music and as far as I know there are no issues with volume.

Mind you I am a newbie to this so for all I know I`m listening to half a system.........